Chiral hydrogen-bonded supramolecular capsules: Synthesis, characterization and complexation of C70

Article abstract of DOI:10.1039/c8cc10152c

Two supramolecular nanocapsules were generated by multi-component self-assembly of the novel bisphosphoric acid (R,R)-6 with suitable bis- and trisamidines. The resulting chiral, hydrogen-bonded capsules are stable even in polar media and at low concentrations and can be employed for the binding of C₇₀-fullerene in solution.

Full text of DOI:10.1039/c8cc10152c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Chiral hydrogen-bonded supramolecular capsules: Synthesis,
characterization and complexation of C70
Martha Kohlhaas,a Manfred Zähres,b Christian Mayer,b Marianne Engeser,c Christian Mertend and
Received 00th January 20xx,
Accepted 00th January 20xx
Jochen Niemeyer*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two supramolecular nanocapsules were generated by multi- building blocks with hydrogen-bond donor or acceptor groups
component self-assembly of the novel bisphosphoric acid (R,R)-6 are synthetically easily available.
with suitable bis- and trisamidines. The resulting chiral, hydrogen- After the pioneering works by Rebek on self-assembled,
bonded capsules are stable even in polar medium and at low homodimeric hydrogen-bonded capsules,10 more complex
concentrations and can be employed for the binding of C70- multi-component supramolecular capsules11 have been been
reported, some of which are even stable in polar media.12 The
introduction of chirality into such hydrogen-bonded aggregates
was achieved both by using (achiral) dissymmetric subunits,
leading to capsules with supramolecular chirality,13 and by the
use of chiral subunits in the self-assembly process. Homomeric
chiral capsules based on such chiral subunits were reported by
Ungaro14 and Mastalerz15, who employed chiral calix[4]arene or
isatin-derivatives that self-assembled into dimeric and
fullerene in solution.
Supramolecular capsules have emerged as a highly interesting
class of compounds due to their applicability in host-guest
chemistry (i.e. as „nanocontainers“) or in supramolecular
catalysis (i.e. as „nanoreactors“).1 More recently, there has
been growing interest in the design and application of chiral
supramolecular capsules,2,3 since these can be used in
stereoselective guest-binding4 or even in enantioselective
catalysis.5
The synthesis of supramolecular capsules usually takes place
under thermodynamic control by self-assembly of suitable
builiding blocks. Different types of supramolecular interactions,
such as metal-ligand interactions,6 hydrogen-bonding1b, 7 or
halogen-bonding8 have been employed for the generation of a
large variety of supramolecular capsules. For an application in
nonpolar media, the use of hydrogen-bonded capsules is
especially interesting for several reasons: Hydrogen-bonds are
thermodynamically relatively strong but kinetically labile,9
which allows guest-complexation and decomplexation in a
dynamic fashion. From a structural point of view, the
directionality of the hydrogen-bond allows the facile design of
supramolecular capsules with different shapes, and suitable
octameric capsules, respectively. Yashima reported
a
heteromeric, multi-component assemly of chiral bisamidines
with tri- and tetracarboxlic acids, which gave rise to the
corresponding cylindrical capsules based on strong amidinium-
carboxylate interactions.16
Surprisingly,
1,1´-binaphthyl-2,2´-diol
(BINOL),
which
represents one of the most widely used chiral building blocks,
has not been used for the construction of chiral hydrogen-
bonded supramolecular capsules so far. Based on the successful
use of BINOL for the construction of covalent17 and metallo-
supramolecular18 capsules, we envisaged the use of chiral
1,1´-binaphthyl-phosphoric acids for the generation of multi-
component hydrogen-bonded capsules. In this account, we
report on the synthesis of a chiral bisphosphoric acid, which
yields well-defined, supramolecular capsules upon mixing with
suitable bis- and tris-amidines. The capsules feature large
internal cavities and were succesfully applied for the
complexation of C70-fullerene in solution.
For the construction of our capsules, we designed a linear
bisphosphoric acid building block (6) featuring two 1,1´-
binaphthyl-phosphoric acids. In combination with a trigonal-
planar benzene-1,3,5-trisamidine (7), this would result in a the
C3-symmetric molecular capsule (1), while the use of a linear
benzene-1,4-bisamidine (8)would give rise to an aggregate with
a rectangular shape (2).
a. Institute of Organic Chemistry and Center for Nanointegration Duisburg - Essen
(CENIDE), University of Duisburg-Essen, Universitätsstrasse 7, 45141 Essen,
Germany.
b. Department of Chemistry and Center for Nanointegration Duisburg-Essen
(CENIDE), University of Duisburg-Essen, 45141 Essen, Germany.
c. Kekulé-Institute for Organic Chemistry and Biochemistry, University of Bonn,
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
d. Ruhr-Universität Bochum, Organische Chemie II, Universitätsstrasse 150, 44801
Bochum, Germany.
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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The synthesis of the bisphosphoric acids was carried out based the capsules show symmetrical species in solution (see SI fig.
View Article Online
DOI: 10.1039/C8CC10152C
on a 4,4´-dibromo-functionalized BINOL-derivative (see fig. 1 S37 - S44). Upon formation of the capsules, the most disctint
and SI). Twofold Suzuki-coupling with 4-(TIPS-ethynyl)benzene changes are observed for the benzamidines 7/8 which undergo
boronic acidgave doubly TIPS-protected precursor (R)-3. Mono- two- or threefold-phosphate-amidinium pairing. This leads to a
deprotection
substoichiometric amounts of TBAF in a carefully controlled 7.78/7.44 ppm for 1/2, c.f.
reaction to give (R)-4, which then underwent Glaser-coupling desymmetrization of the N-ethyl-groups, resulting in separated
and phosphorylation to give the bisphosphoric acid (R,R)-6 (see signals (e. g. CH2-groups: H = 3.15 and 3.02 / 3.13 and 3.10 ppm
fig. 1). All compounds were fully characterized by standard for 1/2, c.f. H = 2.99/2.96 ppm for 7/8).
was
achieved
by
treatment
with strong downfield-shift for the aromatic CH-groups (H
=

H = 7.11/7.16 ppm for 7/8) and to a


spectroscopic and analytical techniques (see SI). For the Unfortunately, attempts to observe the intact capsules by mass
synthesis of the supramolecular capsules, (R,R)-6 was mixed spectrometry were unsuccessful, which might be attributed to
with 7/8 in the correct stoichiometries. This leads to immediate dissociation of the aggregates at the lower concentrations used
formation of the hydrogen-bonded phosphate-amidinium- for ESI-mass spectrometry. Alternatively, protonation/ deproto-
pairs, resulting in the desired capsules (all-R)-1 and (all-R)-2, nation of the neutral capsules during the ESI process could
respectively (see fig. 1).
destabilize the hydrogen bonds and lead to fragmentation into
the detected smaller aggregates, i.e. [6+7/8+H]+, [6+72+H]+,
[62+72+H]+ and [6+82+2H]2+ (see SI fig. S6 - S12).
However, further proof for formation of the desired capsules
was obtained by DOSY and NOESY-NMR (see fig. 2 and SI fig. S13
- S17). Identical diffusion coefficients were observed for the
bisphosphate and the bis-amidinium / tris-amidinium-subcom-
ponents, respectively. The diffusion coefficients of the capsules
(D = 1.57/2.36*10-10 m2 s-1 for 1/2, are significantly smaller than
those of their individual components (D = 3.21/6.02/8.83*10-10
m2 s-1 for 6/7/8). The diffusion coefficient for 1 is in excellent
agreement with the predicted value (DCalc = 1.58*10-10 m2 s-1),
based on its calculated structure. The predicted diffusion
coefficient for 2 (DCalc = 1.84*10-10 m2 s-1) is slightly smaller than
the observed value, which we attribute to its highly porous
structure, which is not taken into account in the calculation.
Fig. 2: NOESY- and DOSY-spectra of the [3+2]capsule (all-R)-1 (NOESY: 600/600 MHz,
DOSY: 500 MHZ, both 4 MM capsule, [D1]-chloroform/[D4]-methanol = 8/2 (v/v), 298 K.
Table 1: Diffusion coefficients and hydrodynamic radii as determined per DOSY-NMR and
calculated molecular radii.
DOSY-results(a)
D [10-10 m2 s-1]
3.21 ± 0.06
6.02 ± 0.22
8.83 ± 0.01
1.57 ± 0.08
2.36 ± 0.03
Calculated(b)
Compound
D [10-10 m2 s-1]
Fig. 1: Synthesis of the supramolecular capsules (all-R)-1 and (all-R)-2. Top: Synthetic
(R,R)-6
7
8
(all-R)-1
(all-R)-2
-
-
-
scheme,
bottom:
Calculated
structures
of
the
capsules
[B3LYP/6-
Components
Capsules
31G(d,p)/IEFPCM(chloroform), nonpolar hydrogens omitted for clarity]. Reagents and
conditions: i) 4-(TIPS-ethynyl)benzene boronic acid, (dppf)PdCl2, Na2CO3, 85 °C,
DME/Na2CO3 (2 M), 91%; ii) TBAF (0.7 equiv.), 25 °C, RT, THF, 34% (alongside with 57%
reisolated (R)-3); iii) (PPh3)2PdCl2, CuI, 25 °C, THF/CHCl3, 98%; iv) POCl3, pyridine, 60°C,
then H2O, 26%; v) [D1]-chloroform/[D4]-methanol = 8/2 (v/v), 25 °C, 5 min.
1.58
1.84
a) in [D1]-chloroform/[D4]-methanol (8/2 v/v) at 298 K, b) calculated based on the
DFT-optimized structures using the Stokes-Einstein equation and the
approximation for cylindrical particles.
Formation of both capsules can easily be followed by NMR-
1
spectroscopy. As expected, the H, 13C and 31P NMR spectra of
2 | J. Name., 2012, 00, 1-3
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Based on DOSY-measurements at different concentrations, the derivative is soluble in [D1]-chloroform/[D4]-mVeietwhaArntioclel Oanlninde
1
supramolecular capsules seem to be stable over a wide possesses H NMR signals, allowing analDyOsiIs: 1o0f.10th39e/Ch8oCsCt1-g0u15e2sCt
concentration range (0.1 - 4 mM capsule concentration). In the interaction by titration-experiments and NMR-analysis.
NOESY-NMR, clear crosspeaks are observed between the We first analyzed the host-guest interaction by fluorescence-
amidinium-ethyl groups and those bisphosphate CH-groups in spectroscopy. While the empty capsules are strongly
spatial proximity (H-3, H-13, H-22, H-23, H-30, H-31, for fluorescent (max = 410 nm), the fluorescence is strongly
numbering see fig. 1), most clearly observable for the larger quenched in the presence of C70-IPH.
capsule (all-R)-1 (see fig. 2).
Fluorescence titrations of (R,R)-6 (as a control) and of the
These experimental results are in excellent agreement with the capsules (all-R)-1/2 clearly show that both dynamic and static
DFT-calculated structures of the capsules (see. fig. 1). The quenching take place in all cases, as indicated by an upward
cylindrical complex 1 is C3-symmetric with a dimensions of ca. curvature in the Stern-Volmer plots (see fig. 3). In presence of
5.2 x 2.0 nm (height x diameter), while capsule 2 adopts an three equivalents of C70-IPH, the fluorescence of (R,R)-6 is
overall rectangular geometry, with dimensions of ca. 5.2 x 2.7 quenched by 70%, while quenching is significantly stronger in
nm (height x width). In both cases, the capsules possess one case of the capsules 1/2 (98%/80%, respectively).
central cavity, flanked by the butadiyne-linkers, plus two The host-guest interactions could be analyzed by deconvolution
additional peripheral cavities, flanked by the TIPS- of both quenching contributions (see table 2), composed of a
ethynylphenyl-groups.
linear dynamic quenching component (described by KSV) and a
In the minimized structures, both (all-R)-1 and (all-R)-2 adopt a static quenching component based the equilibrium formation of
right-handed helical structure, with the butadiyne-linkers a nonfluorescent host-guest complex (described by Ka).
featuring a tilt angle of ca. 37.9°/39.2° (for 1/2) as opposed to Comparative fitting using different host-guest stoichiometries
0° in a non-tilted geometry (see SI fig. 1 and SI fig. S2 - S4 for showed that a 1:2 host:guest complex is formed.
further views on the structures). Interestingly, the
corresponding diastereomeric left-handed helix of (all-R)-1 was
not stable on DFT level and any attempt to optimize such
geometry converted it to a right-handed helical structure. In
contrast, pre-optimizations of (all-R)-1 performed on OPLS force
field level yielded two stable minima for the diastereomeric
structures, actually favoring the left-handed helical geometry
by about 3 kcal/mol. These contradicting results of the
geometry optimizations may suggest a certain conformational
flexibility which could explain the lack of any additional Cotton-
effects in either ECD and VCD spectra (see SI fig. S5). However,
it also clearly shows geometry optimizations of such
supramolecular assemblies, which are typically carried out at
force field levels, can easily lead to wrong conclusions about
supramolecular chirality.
Since both supramolecular capsules possess multiple internal
Fig. 3: Stern-Volmer plots of the titrations of (R,R)-6, (all-R)-1 and (all-R)-2 with C70-IPH
[all: 298 K, [D1]-chloroform/[D4]-methanol (8/2 v/v), initial concentration of acid/capsule
100 M, Exc = 350 nm, em = 410 nm]. Box: Structure of C70-IPH.
cavities, we evaluated their use in host-guest chemistry. Based
Table 2: Stern-Volmer constants and association constants of C70-IPH to the different
on the cavity sizes (ca. 0.7-1.0 nm3) we envisaged that both
capsules should be suitable for encapsulation of fullerenes, such
as C70 (volume of ca. 0.6 nm3).19 The supramolecular
encapsulation of fullerenes such as C60 and C70 has been realized
with different types of host molecules:20 Purely organic
macrocyclic receptors, such as cyclodextrins,21 but especially
systems containing multiple arene- or heteroarene22 units
(including BINOL-based macrocycles23) have been reported. In
addition, TTF- and metalloporphyrin-based24 receptors have
hosts (6/1/2) based on fluorescence titrations.a
Dynamic
quenching
KSV [M-1]
Static quenching /
complex formation
Ka [M-1]
Host
(R,R)-6
4700 ± 770
20 300 ± 690
5000 ± 60
2100 ± 1500
20200 ± 4500
4600 ± 2100
[3+2] capsule (all-R)-1
[2+2] capsule (all-R)-2
(a) Based on dynamic and static quenching, static quenching based on 1:2 complex
been
described.
More
recently,
the
use
of
stoichiometry in a noncooperative binding mode (K11 = 4* K12, only K11 reported).
metallosupramolecular cages for fullerene encapsulation has
emerged,25 but only few examples for hydrogen-bonded
supramolecular fullerene-hosts have been reported.26
We first tested the complexation of unfunctionalized C70 in
combination with 1 and 2: In a mixture of [D1]-chloroform and
[D4]-methanol (8/2 v/v), C70 was solubilized in the presence of
both capsules, while it was completely insoluble in their
absence. For easier characterization, we then resorted to using
the monofunctionalized derivative C70-IPH (see fig. 3). This
The Stern-Volmer constant is similar for (R,R)-6 and (all-R)-2
(4700-5000 M-1), but higher for (all-R)-1 (20 300 M-1), probably
reflecting the larger size of 1 in solution. Static binding takes
place in all cases: The association constant for binding of C70-IPH
to the phosphoric acid (R,R)-6 amounts to 2100 M-1, while
binding to the capsules is significantly stronger: The association
constant towards the [2+2]-capsule (all-R)-2 amounts to
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4600 M-1, while the three-dimensional cavities in the [3+2]-
capsule (all-R)-1 enable even stronger binding (Ka = 20 200 M-1),
as reflected in the strong curvature of the Stern-Volmer plot.
For NMR-studies, we prepared capsule/C70-IPH mixtures at
higher concentrations. It was found that both capsules
solubilize ca. 2.6 equivalents of C70-IPH (i.e. 10.4 mM C70-IPH @
4 mM capsule), which strongly exceeds the solubility of C70-IPH
in the absence of capsule (ca. 5 mM). Based on the strong host-
guest interaction, the encapsulation of C70-IPH into the
[3+2]capsule (all-R)-1 can be proven by NOESY-NMR, which
shows clear crosspeaks between the aliphatic protons of C70-IPH
and the aromatic protons of the capsule (see SI fig. S49). This
was not observed for the mixtures of C70-IPH with (R,R)-6 or
(all-R)-2, underpinning the observed differences in association
constants.27
Im summary, we have reported on the synthesis and application
of novel chiral supramolecular capsules, based on the
bisphosphoric acid (R,R)-6. Simple mixing with the
corresponding bis- and trisamidines leads to spontaneous
formation of the chiral capsules (all-R)-1 and (all-R)-2, which are
stable even in polar solvent and at low concentrations based on
strong phosphate-amidinium-pairing. Due to their large internal
cavities, both capsules were successfully applied for the
encapsulation of C70-IPH in solution, showing that especially the
[3+2]capsule (all-R)-1 is a suitable host for C70. Further
experiments for the application of the chiral capsules in
stereoselective host-guest chemistry are currently underway in
our laboratories.
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Acknowledgements
M. K. would like to thank Evonik Industries AG for a PhD-
fellowship. Funding by the Fonds der Chemischen Industrie and
the DFG is gratefully acknowledged. We thank Dr. C. Strassert
(University of Münster) for helpful discussions. J. N. would like
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Conflicts of interest
There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8CC10152C
Hydrogen-bonded nanocapsules were generated by multi-component self-assembly of phosphoric
acids and amidines and could be used as hosts for C70.